It was fluttering over by the tennis courts, corner of Russell Boulevard and Howard Way, on the University of California, Davis campus.

Shapiro, professor of ecology and evolution, writes on his popular "Butterfly World" website that these yellow sulphur butterflies (Colias eurytheme) reach "very high densities in alfalfa fields in midsummer to autumn."

When the alfalfa is cut, they "may emigrate en masse, even flooding into cities," he says.

"Although it is a significant alfalfa pest, this butterfly overwinters as a larva almost entirely in annual vetch at low altitudes, and colonizes alfalfa only as the vetch senesces in May-June," Shapiro points out. "Aside from alfalfa and annual vetches, it also breeds on a variety of clovers and sweet clovers and occasionally on lupines. It seems certain that the planting of alfalfa has greatly increased its range and abundance."

With these colorful butterflies, they don't stay linger long during their flight of fancy. You have to shoot on the run. Aim, pre-focus, shoot.